Numerous circuits which can be used to drive an ultrasonic transducer at useful power levels are known. These transducers are commonly made from a piezoelectric ceramic material which exhibits electromechanical resonance effects typical of many piezoelectric devices. When such piezoelectric devices are operated at one of their natural resonance frequencies, greatly improved electrical to mechanical power conversion can be accomplished, especially when the resulting vibrations are amplified using a suitable horn.
A known application of ultrasonic waves is in the atomization of liquids, particularly fuel oil. Specifically, a piezoelectric transducer is constructed so that fuel is allowed to flow in the form of a film over an atomizing surface of its horn. When the transducer is excited at one of its natural resonance modes with sufficient amplitude, the film of fuel oil that covers the horn is propelled from the surface in the form of a fog of fine droplets. Such an ultrasonic transducer has applications as a means of atomizing the fuel in an oil burning furnace, replacing, for example, the commonly used high pressure spray nozzle.
During atomization, and for any fixed system efficiency, there is a definite relationship between the viscosity and flow rate of the liquid and the minimum energy required to sustain atomization. Increasing energy is therefore required as the viscosity and/or the flow rate of the liquid increase. For any given energy or power level, excessive liquid viscosity or flow rate will cause the atomizer to flood and atomization will stop.
In the case of an ultrasonic atomizer used for atomizing fuel oil in an oil burner, the necessity to control the air to fuel ratio accurately for optimum operation ensures the fuel flow rate is well defined. However the viscosity of the fuel may vary widely as a result of operation over a wide environmental temperature range or the use of different fuel grades. It is therefore a real possibility that, at times, flooding of the atomizer may occur and so it is a necessary requirement of an ultrasonic generator used to drive such an atomizer that the generator is able to recognize when flooding of the atomizer has occurred and is further able to recover from this condition.
A known method used to sense the occurrence of flooding is to sense if the atomizer is no longer being driven at its chosen resonance frequency. The circuit required to sense this is generally just an extension of the circuit used to find and follow the resonance of the ultrasonic transducer. One type of ultrasonic generators find and follow the transducer resonance frequency by comparing the phase of the driving voltage with the phase of the resulting transducer current, and change the driving frequency until voltage and current are in phase. In these ultrasonic generators it is assumed the atomizer is flooded when the driving voltage and the resulting current fall out of phase. When this occurs, typically the generator is caused to begin sweeping the transducer over a defined range of frequencies until the resonant point is again found. For ultrasonic generators that use another method of resonance detection, namely sensing the frequency where the transducer current is at a maximum (for operation at series resonance) or a minimum (for operation at parallel resonance), then it is assumed that the atomizer is flooded when the current is no longer at the maximum or minimum value. Again, in this case, the generator typically begins sweeping the transducer over a range of frequencies in an attempt to locate the amplitude maximum or minimum and once again establish stable operation.
Another known method used for the sensing of atomizer flooding makes use of the reduction of the "Q" of the resonant system that occurs when the atomizer floods. With this method, when the value of the transducer current drops below a set threshold, the atomizer is assumed to be excessively damped and therefore flooded. Again, typically the generator begins frequency sweeping in an attempt to clear the atomizer of excess liquid and once again find the system resonance.
EP-A-0 340 470 discloses a further method for detecting flooding in an atomizer wherein the sharpness of resonance "Q" of the resonant system is observed by evaluating the edge steepness of the resonance curve. For this purpose the resonant circuit used in this known method does not lock to a resonance frequency but sweeps continuously the excitation frequency between two frequency limits on each side of the resonant frequency. If the resonance is pronounced enough the sweeping takes place between the two frequency limits of a narrow sweeping range. If weak resonance is detected the sweeping takes place between the two frequency limits of a wide frequency range. The steepness of the resonance curve is determined by feeding the voltage drop across a resistor through which the current of the driver output stage of the control circuit flows, to a comparator directly, on the one hand, and via a delay circuit, on the other hand. If the differences between non-delayed voltage and delayed voltage are below a certain threshold, it is assumed that the resonance curve is too weak and the wide sweeping range is switched to. If sweeping over the wide frequency range succeeds in propelling off non-atomized droplets, the edges of the resonance curve become steeper again and sweeping over the narrow frequency range can be resumed.
All the above methods of flooding detection, however, have proven to be unreliable in the detection of atomizer flooding. The main reason for this is their inability to reliably detect a common flooding mechanism.
To elaborate on this, with a typical ultrasonic atomizer the liquid to be atomized is caused to flow through a hole drilled axially along the length of the horn and emerges in the center of the horn face. From there, it flows in a film radially outward on the face of the horn. As it flows outward from the vibrational node at the horn center, it is subjected to increasing acceleration due to the ultrasonic vibrations which are at a maximum at the extreme periphery of the horn face. Normally, before the liquid reaches the periphery, it reaches a point where there is sufficient acceleration to drive it off the horn as a fog of atomized liquid. Thus, atomization primarily occurs in a relatively narrow ring-shaped zone on the atomizer face. The mean radius of this atomization zone relative to the radius of the horn for any given system power level and efficiency is mainly determined by the viscosity of the liquid and its flow rate.
In the case of the atomizer being used as a means of atomizing fuel in a furnace over an extended temperature range, the fuel flow rate as mentioned above is closely controlled, but the fuel viscosity may vary widely. Therefore it is not uncommon for the fuel viscosity at times to be so high that, for a particular power level, the fuel will flow all the way to the edge of the horn face and still will not receive enough energy to propel it from the horn and effect atomization.
When this condition occurs rapidly, the fuel immediately collects at the outer periphery of the horn and the rapid and very substantial increase in damping which immediately occurs causes the generator to lose control of the transducer frequency. This results in a complete loss in atomization with the fuel flowing from the atomizer face in much the same way as if the generator was simply switched off. In this case, the system is so highly damped and/or being driven so far off resonance that the electrical to mechanical power conversion at the transducer is negligible. This abrupt form of flooding is generally detectable by one of the above outlined methods.
A very much more difficult to detect mechanism of flooding occurs when the atomizer slowly begins to flood. Such a case occurs, for example, when the liquid volume slowly increases toward a set flow rate, the magnitude of which exceeds the flow rate for which atomization can be sustained under the conditions of viscosity and power level present. Since it is common to require the use of an impulse damper in the fuel delivery line of some oil burning furnaces for the purpose of smoothing the flow impulses caused by the action of the fuel pump, this gradual increase of fuel flow toward a steady state flow rate will occur in such a system each time the fuel flow is started. This action is due to the nature of the impulse damper which acts as a temporary storage reservoir, opposing any rapid changes in the fuel volume delivery rate. Initially, before full fuel output occurs, the flow rate will be lower than that which will cause flooding, under the present conditions of fuel viscosity and power level. As the flow rate increases, the atomization zone will move closer to the edge of the atomizer horn, and it may reach the very edge of the horn.
When this happens, the atomizer is on the verge of flooding. As the flow rate continues to slowly increase, atomization begins to break down as liquid fuel starts to collect around the rim of the horn. This fuel adds effective mass to the atomizer horn, which begins to cause the transducer's natural resonance frequency to decrease slightly. This is sensed by the generator, also called an excitation circuit here, whose output frequency correspondingly decreases to match the new resonance. This process continues with more fuel building up on the face of the horn and the resonance frequency decreasing until atomization is halted completely, and a hemispherically shaped mass of fuel, supported by standing waves, builds and is held on the entire face of the atomizer horn. Excess fuel supplied by the pump now simply runs off, leaving a very stable system, with a somewhat lower "Q" due to the fluid damping, operating at a new somewhat lower natural resonance frequency due to the added mass of the fluid. Even if the fuel flow is stopped, the mass of fuel will remain attached to the horn, and the system will continue to operate uselessly at its new resonance point for many minutes.
The atomizer is now completely flooded, no atomization is taking place, yet the methods of flooding detection mentioned above are unable to detect this because the system is indeed at resonance and the system "Q" is not unreasonably low. The only way to clear this large amount of excess fuel is to either switch off the system, or to quickly drive the frequency to a much different value, such as the minimum frequency in the range. In either case, this eliminates the standing waves that support the excess fuel, and it immediately falls away.